The present invention relates generally to the field of process control instruments. More particularly, the present invention relates to a new data bus system and protocol for communication between master control nodes and peripheral devices in process control field instrument operating in accordance with the standard Motorola.TM. SPI protocol.
Process control instruments such as transmitters and controllers are used to monitor and/or control industrial processes. In some recent designs, these process control instruments include a number of internal devices or components such as a microprocessor which functions as a master node and a number of peripheral electronic devices which act as slave nodes. The peripheral electronic devices include, for example, pressure sensors for measuring process pressures, temperature sensors for measuring process temperatures, valves for controlling a process, digital-to-analog and analog-to-digital converters for converting data transferred between analog and digital devices, communication links for transmitting and receiving information over a two-wire process control loop, memory devices for storing instructions or monitored process results, and displays for providing a graphical representation of some process related information. Typically, all of these devices are internal to the instrument or in close proximity to the instrument. The instrument can include any combination of the above mentioned slave nodes (i.e., peripheral devices). The various slave nodes in the process control instrument communicate on a data bus which connects each of the peripheral devices to the master node.
Frequently, the master node in a process control instrument is a Motorola or compatible microprocessor or microcontroller. For example, the Motorola 68HC05 and 68HC11 microcontrollers are commonly used as the master node in process control instruments. These and other microcontrollers are designed to operate in accordance with the standard Serial Peripheral Interface (SPI) bus communication protocol which is defined by Motorola in the M68HC11 Reference Manual which is hereby incorporated by reference. While use of microprocessors or microcontrollers which operate in accordance with the SPI protocol is in many circumstances advantageous, the SPI protocol itself introduces a number of disadvantageous data bus requirements and has a number of shortcomings. For example, in process control instruments, the size of the devices can be critical. Therefore, smaller printed circuit boards (PCB's) are preferred for inside the process control instrument. However, with the SPI protocol, a separate chip select (CS) data bus line is required for each peripheral device which is to be connected to the master node. In addition to requiring more PCB space for the additional CS bus lines, this requirement of the standard SPI protocol also requires additional pins on the master node chip. Another disadvantage of the SPI protocol relates to the difficulties of adding or changing the master node or slave nodes. Typically, each device which communicates on the process control instrument data bus has a predetermined address. Adding, removing or interchanging nodes requires that the bus and/or addressing be customized to accommodate the change.
As illustrated above, it is apparent that the SPI protocol provides a number of disadvantages, particularly in data buses within process control instruments. The present invention overcomes these and other disadvantages of the standard SPI communications protocol.